Catalyst comprising amorphous NiO on silica/alumina support and process for butene dimerization

ABSTRACT

The present invention provides for a catalyst composition which is effective for use in the production of dimer products and higher olefin products from lower olefins such as propylene and butene in high yields and with an average degree of branching in the dimer products of less than about 1.6 methyl groups per molecule, generally in the range of from about 1.0 to 1.4 methyl groups per molecule. The present invention also provides a process for producing such dimer and higher olefin products using the catalyst composition of this invention. The catalyst of the invention comprises an amorphous nickel oxide (NiO) present as a disperse substantial monolayer on the surfaces of a silica (SiO 2 ) support, which support also contains minor amounts of an oxide of aluminum, gallium or indium such that the ratio of NiO to metal oxide present in the catalyst is within the range of from about 4:1 to about 100:1. The catalyst may be prepared by precipitating a water insoluble nickel salt onto the surfaces of a silica support which has been impregnated with the metal oxide or onto a silica-alumina support which has been dealuminized such that the resulting nickel oxide/alumina ratio will fall within the ranges set forth above. The catalyst may then be activated by calcination in the presence of oxygen at a temperature within the range of from about 500° to 700° C.

This is a division of application Ser. No. 644,998, filed Jan. 22, 1991,now U.S. Pat. No. 5,169,824.

FIELD OF THE INVENTION

The present invention relates to catalyst materials useful for theoligomerization of C₃ and C₄ olefins which catalysts comprise amorphousNiO present as a substantial monolayer on the surfaces of a silicasupport, and which catalyst further contains minor quantities of atrivalent metal oxide selected from the group consisting of aluminumoxide, gallium oxide and indium oxide.

DESCRIPTION OF RELATED ART

The conversion of C₃ or C4 olefins into dimers and hydrocarbons ofhigher molecular weight using catalysts comprising nickel supported onsilica or silica/alumina supports is known in the art. Dimers such ashexenes and octenes so produced are particularly useful for conversionby the well known oxo alcohol process into the corresponding heptyl andnonyl alcohols which may be used in the production of plasticizers,lubricating oil additives, detergents, defoamers and similar products.

Octene dimers are particularly useful in the manufacture of plasticizeralcohols. These dimers generally comprise a mixture of octene isomershaving a varying degree of side chain methyl substitution per moleculepresent in the mixture.

N-octenes, for example, contain no side chain methyl groups,methyl-heptenes contain 1 side chain methyl group; dimethylhexenescontain 2 side chain methl groups; and trimethylpentenes contain 3 sidechain methyl groups. A mixture of isomeric octenes having an average ofless than about 1.6 side chain methyl groups per molecule is especiallysuitable for conversion into nonyl alcohols and gives rise to higheroxonation yields and better quality plasticizer alcohols.

It is known to dimerize olefins by contact with a nickel oxide catalystat elevated temperature. For example, U.S. Pat. No. 3,649,710 describesa process in which butene and propylene are first pre-treated and thenco-dimerized by passing over a nickel oxide catalyst. The referenceteaches that the pre-treatment of the olefin substantially improves thelife of the catalyst. U.S. Pat. No. 3,658,935 discloses a process forpreparing propylene and n-butene dimers by contacting the feed underreaction conditions with a catalyst comprising a silica alumina gelcontaining 10 to 45% alumina and 0.1 to 35% nickel. In addition, U.S.Pat. No. 3,557,242 discloses a process for copolymerizing isobutene anda lower olefin using a catalyst comprising jointly coprecipitatednickel, silicon and aluminum oxide species wherein the catalyst containsfrom 2 to 12 wt. % nickel, 0.4 to 5 wt.% aluminum, 0.05 to 0.8 wt. %alkali metal and up to 5 wt.% coprecipitated magnesium oxide.

British Specification No. 1069296 discloses the production of dimersfrom olefins such as butene by contact with a catalyst containingaluminum and nickel ions on a silica support at temperatures up to 400°C. British Specification No. 1215943 discloses the dimerizing of olefinsincluding butenes by contact with the same kind of catalyst which isactivated in a slightly different manner. The catalyst used in thesespecifications differs from those used in the U.S. Patents referred toabove primarily by including only a small proportion of nickel in thecatalyst composition, generally less than 10% by weight based on theweight of silica gel.

In addition, U.S. Pat. No. 2,581,228 discloses a catalyst useful forpolymerizing olefins which comprises a silica gel impregnated with anickel salt and an aluminum salt such that the activated catalystcontains from about 0.1 to 35 wt. % nickel in the form of nickel oxideand from about 1 to 10 wt. % alumina based on the weight of silica gel.

Such known catalysts and methods for dimerizing lower olefins such asbutene into higher olefins such as octene suffer from one or moredisadvantages. The percent conversion of olefin to dimer and higherproducts may be low and the conversion per pass over the catalyst usedto form the more valuable dimer products such as octenes is too low,generally less than 50%. While the yield of octenes may be increasedusing dimerization process conditions including relatively low spacevelocity (longer catalyst contact time) and higher temperatures, it isfound that the octenes produced are more highly branched, having anaverage content of side chain methyl substituent groups in excess ofabout 2. At higher space velocities and lower dimerization temperatures,the degree of branching is reduced but at the expense of significantlylower yields of octene products. This means that significant portions ofthe product must be refined for recycle to the dimerization reactorwhich diminishes the economy and efficiency of the process.

SUMMARY OF THE INVENTION

The present invention provides for a catalyst composition which iseffective for use in the production of dimer products and higher olefinproducts from lower olefins such as propylene and butene in high yieldsand with an average degree of branching in the dimer products of lessthan about 1.6 methyl groups per molecule, generally in the range offrom about 1.0 to 1.4 methyl groups per molecule. The present inventionalso provides a process for producing such dimer and higher olefinproducts using the catalyst composition of this invention. The catalystof the invention comprises an amorphous nickel oxide (NiO) present as adisperse substantial monolayer on the surfaces of a silica (SiO₂)support, which support also contains on the surface minor amounts of anoxide of aluminum, gallium or indium such that the ratio of NiO to metaloxide present in the catalyst is within the range of from about 4:1 toabout 100:1. The catalyst may be prepared by precipitating a waterinsoluble nickel salt onto the surfaces of a silica support which hasbeen impregnated with the metal oxide or onto a silica-alumina supportwhich has been dealuminized such that the resulting nickel oxide/aluminaratio will fall within the ranges set forth above. The catalyst may thenbe activated by calcination in the presence of oxygen at a temperaturewithin the range of from about 500° to 700° C.

DETAILED DESCRIPTION OF THE INVENTION

In order to achieve improved yields of dimer olefins having an averagedegree of branching of less than about 1.6 in accordance with thepresent invention, it is essential that the silica-based catalystcontain NiO in the form of an amorphous layer dispersed uniformly on thesurface of the silica in quantities such that a substantial monolayer isformed. The presence of the nickel compound as a monolayer maximizes thereaction of the nickel with the underlying silica when the catalyst iscalcined to form a less reducible layer-like nickel-silicate or hydroxysilicate species. This requires that the NiO precursor be deposited onthe surface of the silica carrier by precipitation techniques such thatit is uniformly dispersed on the surface of the silica as a continuouslayer, in contrast to other deposition techniques such as impregnationor ion exchange where the NiO decomposition product is present inisolated regions of the silica or in the form of bulklike agglomerates.In these latter forms, the NiO precursor more readily reduces toelemental nickel when the catalyst is activated by calcination, whichgives rise to a catalyst which is not active or less active as adimerization catalyst.

The quantity of dispersed NiO required to form a substantial monolayeron the surface of the SiO₂ generally ranges from about 0.07 to about0.12% by weight per square meter per gram (m² /g) of surface area in thesupport. Calculations show that an ideal NiO monolayer is formed at aquantity of about 1% of dispersed NiO per 11 m² /g of SiO₂ surface.

Silica gels useful as support materials for the purpose of thisinvention may generally have a surface area within the range of fromabout 100 to about 450 m² /g, more preferably from about 200 to about 40m² /g. Thus, the broad range of NiO content is from about 7 to about 55%by weight and the more preferred range is from about 20 to about 40% byweight, depending on the surface area of the particular support utilizedin preparing the catalyst For the most preferred supports having asurface area of about 300 m² /g, the NiO content generally ranges fromabout 21 to about 35% by weight, with 28% by weight NiO content beingmost preferred.

The silica gel may be in dry granular form or in the hydrogel form priorto precipitation of the NiO precursor compound on the surfaces thereof.Silica gel may be prepared by mixing a water-soluble silicate such assodium or potassium silicate with a mineral acid to effect formation ofa silica hydrogel which is then water washed to remove soluble ions. Theresulting hydrogel may then be partly or totally dried.

The NiO precursor compound is most readily precipitated onto the surfaceof the silica gel in the form of a water-insoluble salt such as nickelcarbonate, nickel phosphate or nickel hydroxide. The water insolublesalt is preferably generated in-situ by forming an aqueous mixture ofthe silica gel and a water soluble nickel salt such as nickel nitrate,nickel sulfate, nickel carbonylate, nickel halide or the like, and thenadding a base to this mixture to form the insoluble salt. The waterinsoluble nickel salt which is formed is caused to precipitate in finelydivided form within the interstices and on the surface of the silicagel. The treated silica gel is then recovered, washed several times anddried.

A second essential component in the catalyst of the invention is anoxide of a trivalent metal selected from the group consisting ofaluminum, gallium and indium. Although the NiO or silica catalystdescribed above is active for olefin dimerization, it is found todeactivate quickly, probably as a consequence of the formation of largeoligomers which remain attached to the surface and act as cokeprecursors. The presence of a small amount of the trivalent metal oxidewithin the catalyst yields acid sites which tend to retard largeoligomer formation and thereby promote catalytic activity.

The tri valent metal oxide may be incorporated into the silica gel byany suitable technique such as a precipitation method described above orby direct impregnation in the form of a water soluble salt. It ispreferred to impregnate the silica gel in aqueous solution by theaddition of a water soluble metal salt, such as the nitrate, chloride orsulphate, followed by drying and calcination to reduce the metal salt tothe oxide form. The resulting silica gel activated with the trivalentoxide is then further treated as described above to incorporate the NiOlayer onto the silica-trivalent metal oxide support.

Yet another method for providing the silica-trivalent metal oxidesupport is to utilize a silica/alumina, silica/gallia or silica/indiagel as the starting material. However, the content of the metal oxide(for example, alumina) present in the support of the present inventionmust be low in comparison with the content of NiO, as hereinafterpointed out, and it may therefore be necessary to dealuminizesilica/alumina gel of relatively high alumina content, i.e., above about5% by weight, to reduce the content of alumina. This may readily beaccomplished by techniques known in the art such as by extraction of thealuminum with an organic or inorganic acid such as nitric acid, sulfuricacid, hydrochloric acid, chloroacetic acid, ethylene diamine tetraaceticacid and like materials. Extraction may be accomplished by adding theacid to an aqueous dispersion of the aluminosilicate followed bystirring, decantation and washing with water. The process may berepeated one or more times until the desired alumina content isachieved. The solids are then dried, calcined and further treated asdescribed above to incorporate the NiO layer onto the silica/aluminasupport.

The content of trivalent metal oxide with respect to the content of NiOpresent in the silica support is critical in order to achieve theimproved results in terms of dimer yield and minimum average methylbranching present in the dimer product. Where the content of trivalentmetal oxide is to low, i.e., above a NiO to trivalent metal oxide ratioof about 100 to 1, then the yield of dimer falls off and the catalysttends to deactivate quickly. Where the content of trivalent metal oxideis too high, i.e., below a NiO to trivalent metal oxide ratio of about 4to 1, then the yield of dimer also falls off and the average content ofmethyl branching in the dimer product increases. In the preferredembodiment of the invention, the content of trivalent metal oxide issuch that the ratio of NiO to trivalent metal oxide falls within therange of from about 4:1 to 30:1, more preferably from about 5:1 to 20:1,and most preferably from about 8:1 to about 15:1.

Thus, in the most preferred embodiments of the invention, the catalystcontains from about 21 to 35% by weight of NiO and from about 1 to 5% byweight of trivalent metal oxide, more preferably from about 1.5 to 4% byweight trivalent metal oxide, based on the total weight of NiO,trivalent metal oxide and SiO₂.

Olefins which may be polymerized using the catalyst of the presentinvention include propylene and butene. The process and catalyst isparticularly effective for the dimerization of butene to form a mixedpolymerization product composed mainly of octenes. The butene feedgenerally comprises a mixture of butene isomers which are predominantlya mixture of n-butene-1, trans-butene-2 and cis-butene-2. For example, afeed referred to as trans-butene is one where the predominant butenecomponent is trans-butene-2.

The polymerization may be carried out in either the liquid or gas phase.Temperature conditions include a temperature of from about 150° C. toabout 275° C. and, in the gas phase, a liquid hourly weight feed rate ofbutene over the catalyst of from about 0.4 to about 1.8 h⁻¹, preferablybetween 0.6 and 0.7 h⁻¹. In the liquid phase, pressures to insuresubstantial liquid phase operation should be maintained. Residence timein the liquid phase may generally range from about 0.1 to 2 hours ormore, with residence time (RT in hours) being defined as: ##EQU1## Wherethe polymerization reaction is conducted in the liquid phase and thecatalyst is mixed with the olefin monomers, it is preferred that theratio of monomer to catalyst be in the range of from about 2:1 to about8:1, more preferably from about 4:1 to about 6:1. In cases where thepolymerization is conducted under pressure near, at or above thecritical temperature of the olefin monomer, it is often desirable toinsure that the liquid phase is maintained by carrying out the reactionin the presence of an inert higher boiling hydrocarbon such as a normalparaffin or cycloparaffin.

Butenes suitable for use in the present invention are commerciallyavailable from petroleum refinery operations. Such butenes should notcontain more than 1.5% of isobutenes, because isobutene tends to formproducts with a high degree of branching. Preferably the butenes consistsubstantially entirely of 1-butene, cis-2-butene and/or trans-2-butene.The presence of fully saturated hydrocarbons in the feed is notgenerally detrimental, but if the proportion rises above about 80percent by weight the process becomes uneconomic. The presence ofolefins containing more than 4 carbon atoms per molecule should likewisebe avoided because they reduce the selectivity of the reaction toproduce octenes.

The process of the present invention does not require special apparatusand can be operated in any reactor configuration which is capable ofpromoting intimate contact between the olefin feed and the catalyst. Theprocess can be operated batchwise, semi-batchwise, or continuously.

Besides the desired octene mixture, the process of the present inventionproduces minor amounts of C₅₋₇ and C₉₋₁₆ olefins. Of these, the mostimportant are the dodecenes, which in some cases are produced in aweight proportion as high as half that of the octenes. It is usuallyeconomically worthwhile to isolate and separate such by-product olefins.

The following Examples illustrate the invention:

EXAMPLE 1

This example details a two step preparation of a catalyst consisting ofa NiO monolayer deposited at a level of about 28% by weight on a supportcomposed of 97.6% by weight SiO₂ and 2.4% by weight of Al₂ O₃, theweight ratio of NiO to Al₂ O₃ being about 11.7 to 1.

The first step involved preparing the 2.4% Al₂ O₃ 97.6% SiO₂ support.This was prepared by dispersing 100 g 13% Al₂ O₃ - 87% SiO₂ cojelobtained from Davison (Davison SMR-6672 - surface area =300 m² /g) in2000 cc water. Nitric acid was then added with stirring until the pHreached 2.7. The mixture was then filtered and washed several times withdistilled water until the pH reached 5.6. The recovered solid wastreated again with nitric acid/water as described above then washed withwater until the pH reached 5.6. The recovered solid was dried for 16hours at 100° C. and then calcined at 500° C. in flowing air for 16hours.

For the second step, 67.38 g of Ni(NO₃)₂. 6H₂ O was dissolved in 700 ccof water. The solution was heated to 32° C. and 35 g of thealuminosilicate prepared as above was slowly added. To this slurry asolution of 33.69 g of (NH 2CO in 200 cc of H₂ O was added dropwise. Theslurry was stirred for 1/2 hour and filtered. The recovered solid wasreslurried to its original volume twice with water and filtered. Thesolid was dried in a muffle furnace at 120° C. for 16 hours withnitrogen flowing at 200 cc/min. The material was then calcined in themuffle furnace for 1 hour at 232° C., 2 hours at 371° C. and 16 hours at593° C. in air flowing at 200 cc/min.

EXAMPLE 2

This example details an alternative two step procedure for thepreparation of a catalyst consisting of a NiO monolayer deposited at alevel of about 28% by weight on a support composed of 97.5% by weight ofSiO₂ and 2.5% by weight of Al₂ O₃, the weight ratio of NiO to Al beingabout 11.2 to 1.

In the first step, aluminum nitrate monohydrate in 82 cc distilled waterwas added to 39 g SiO₂ (Cab-O-Sil®HS-5-Surface area=325 m² /g). Theimpregnated material was dried overnight in air at 120° C. and calcinedin air for 16 hours at 500° C.

This support (2.5% Al₂ O₃ on SiO₂) was then used to prepare 28% NiO/2.5%Al₂ O₃ on SiO₂ by the same procedure as set forth in the second step inExample 1.

EXAMPLE 3

This example details a two step procedure for preparing a catalystconsisting of an NiO monolayer deposited at a level of about 28% byweight on a support composed of 98% by weight of SiO₂ and 2% by weightof Ga₂ O₃ (gallia), the weight ratio of NiO to Ga₂ O₃ being about 14 to1.

In the first step, 3.14 g of gallium nitrate hexahydrate in 82 cc ofdistilled water were added to 39.2 g of SiO₂ (Carb-O-Sil® HS-5). Theimpregnated material was dried overnight in air at 120° C. and calcinedin air at 500° C. for 16 hours.

This support (2% Ga₂ O₃ on SiO₂) was then in a second step added to 37.8g of nickel nitrate hexahydrate in 378 cc of distilled water andprecipitated with 18.9 g of (NH₄)₂ CO₃ in 114 cc of water by followingthe preparation procedure of the second step of Example 1.

Each of the catalysts produced in Examples 1, 2 and 3 above wereevaluated as dimerization catalysts by the following procedure:

EXAMPLE 4

The calcined catalyst (14.13 g) and 7.22 g of decane were charged into a300 cc stirred autoclave. The autoclave was purged with nitrogen gas,charged with 70.64 g of transbutene-2 and heated under autogeneouspressure at 175° C. for 5 hours. During the 5 hours, intermediatesamples were collected to obtain additional data at lower residencetimes. The residence times (RT) are defined as: ##EQU2##

At the end of the run, the autoclave was cooled to 5° C. and theresulting liquid product and intermediate samples were analyzed by gaschromatography to measure conversion, selectivity to various productsand branchiness of the octene fraction.

Results are reported in Table 1.

As is evident from the data present in Table 1, the catalysts of thepresent invention provide an average conversion of butene into higherproducts in the range of from about 80 to about 87% and weightselectivity towards the production of octenes in the range of from about5% to 62.5%. At the same time, the octenes produced were found to have alow average content of methyl branching in the range of from 1.18 to1.30 methyl branches per mol of octene produced.

COMPARATIVE EXAMPLE 5

This example illustrates that NiO monolayer catalysts containing acontent of alumina outside the range set forth herein and generally asdescribed in U.S. Pat. No. 2,581,228 provide diminished selectivitytowards the production of octene and a higher average content ofbranched octene products.

The catalyst employed in this example was prepared by precipitating a28% by weight monolayer of NiO on a catalyst support consisting of 25%by weight alumina on 75% by weight of SiO₂, having a surface area ofabout 300 m² /g. The NiO monolayer was produced according to the generalprocedure set forth in the second step of Example 1. The resultingcatalyst had a NiO/Al₂ O₃ ratio of 1.1 to 1.

The calcined catalyst was used to polymerize transbutene-2 in accordancewith the procedure of Example 4. Polymerization results are alsoreported in Table 1.

                                      TABLE 1                                     __________________________________________________________________________              RESIDENCE                                                                             %    WT. % SELECTIVITY TO  C.sub.8 BRANCHINESS              CATALYST  TIME (HR.)                                                                            CONV.                                                                              C.sub.8                                                                          C.sub.12                                                                         C.sub.16                                                                         C.sub.20                                                                         C.sub.24                                                                         CRACKING                                                                             CH3/MOL                          __________________________________________________________________________    EXAMPLE 1 0.2     78   58.1                                                                             24.0                                                                             10.1                                                                             4.7                                                                              0.1                                                                              3.0    --                                         0.4     80   57.4                                                                             24.1                                                                             10.3                                                                             4.9                                                                              0.2                                                                              3.1    --                                         0.75    81   54.3                                                                             25.2                                                                             11.6                                                                             5.5                                                                              0.1                                                                              3.3    --                                         1.00    82   52.3                                                                             25.8                                                                             12.2                                                                             5.9                                                                              0.1                                                                              3.7    1.28                             EXAMPLE 2 0.2     83   58.4                                                                             23.8                                                                              9.2                                                                             5.0                                                                              0.4                                                                              3.2    --                                         0.4     84   58.4                                                                             24.2                                                                              9.3                                                                             4.8                                                                              0.4                                                                              3.0    --                                         0.75    86   55.6                                                                             25.3                                                                             10.2                                                                             5.5                                                                              0.4                                                                              3.0    --                                         1.00    86   53.2                                                                             25.9                                                                             10.9                                                                             5.9                                                                              0.4                                                                              3.7    1.30                             EXAMPLE 3 0.2     78   62.5                                                                             23.5                                                                             8.2                                                                              4.1                                                                              0.3                                                                              1.4    --                                         0.4     84   55.6                                                                             26.0                                                                             10.7                                                                             5.6                                                                              0.5                                                                              1.7    --                                         0.75    88   53.3                                                                             26.7                                                                             11.7                                                                             6.6                                                                              0.5                                                                              1.3    --                                         1.00    88   50.6                                                                             27.2                                                                             12.6                                                                             7.4                                                                              0.6                                                                              1.7    1.18                             COMPARATIVE                                                                             0.2     73   49.4                                                                             25.3                                                                             10.1                                                                             4.3                                                                              0.1                                                                              10.9   --                               EXAMPLE 5 0.4     78   48.2                                                                             25.3                                                                             10.4                                                                             4.6                                                                              0.1                                                                              11.4   --                                         0.75    81   44.9                                                                             26.3                                                                             11.5                                                                             5.1                                                                              0.1                                                                              12.1   --                                         1.00    85   42.9                                                                             26.6                                                                             12.3                                                                             5.7                                                                              0.2                                                                              12.4   1.81                             __________________________________________________________________________

As is evident from Table 1, the catalyst of comparative Example 5 gaverise to conversions only slightly below the catalysts of the invention,but lower selectivity towards octene production and a much higher degreeof branching. In addition, higher quantities of lower cracked productswere produced.

As indicated above, catalysts which consist of a monolayer of NiOdispersed on the surface of essentially pure SiO₂ also exhibit goodactivity for the dimerization of butene with good conversion rates, goodyield of octene and a low degree of branching in the octene product, butthe catalyst is found to quickly become inactive during thepolymerization process. This is demonstrated by the following example.

COMPARATIVE EXAMPLE 6

67.38 g of Ni(NO₃)₂.6H₂ O was dissolved in 700 cc water. The solutionwas heated to 32° C. and 35 g Cab-O-Sil SiO₂ was slowly added. To thisslurry a solution of 33.69 g (NH in 200 cc H₂ O was added dropwise. Theslurry was stirred for 1/2 hour and filtered. The recovered solid wasreslurried to its original volume twice with water and filtered. Thesolid was dried in a muffle furnace at 120° C. for 16 hours withnitrogen flowing at 200 cc/min. The catalyst was calcined in a mufflefurnace 1 hour at 232° C., 2 hours at 371° C., and 16 hours at 593° C.in air flowing at 200 cc/min.

The catalyst consisted of a 28% by weight monolayer of NiO on SiO₂having a surface area of 300 m² /g.

The calcined catalyst was used to polymerize transbutene-2 in accordancewith the procedure of Example 4. Polymerization results are reported inTable 2.

The catalyst remaining at the end of this first cycle was reevaluatedfor polymerization activity by the following procedure:

Subsequent to the testing described in Example 4 (designated Cycle 1)the autoclave was cooled and the liquid product was removed by suction.The vessel containing the catalyst was purged for 16 hours with N₂. 70.6g of fresh butene and 7.22 g. of n-decane were added to the autoclavefollowed by heating to the required temperature. The rest of theprocedure is the same as described in Example 4. This testing step isdesignated Cycle 2. Subsequent testing of catalyst from Cycle 2 as alsocarried out by the same procedure, and is designated as Cycle 3.

The average percent conversion of the starting butene feed product overresidence times of 0.2, 0.4, 0.75 and 1.00 hours in cycle 2 and cycle 3was determined for the catalysts prepared in accordance with Examples1-3 and compared with the average percent conversion over the sameresidence times for the catalyst prepared in accordance with thisExample in cycle 2 and cycle 3.

                                      TABLE 2                                     __________________________________________________________________________              RESIDENCE                                                                             %    WT. % SELECTIVITY TO  C.sub.8 BRANCHINESS              CATALYST  TIME (HR.)                                                                            CONV.                                                                              C.sub.8                                                                          C.sub.12                                                                         C.sub.16                                                                         C.sub.20                                                                         C.sub.24                                                                         CRACKING                                                                             CH3/MOL                          __________________________________________________________________________    COMPARATIVE                                                                             0.2     70   65.9                                                                             21.3                                                                             6.7                                                                              4.3                                                                              0.2                                                                              1.6    --                               EXAMPLE 6 0.4     74   63.2                                                                             22.5                                                                             7.4                                                                              4.6                                                                              0.2                                                                              1.8    --                                         0.75    78   62.2                                                                             23.4                                                                             7.9                                                                              4.9                                                                              0.2                                                                              1.3    --                                         1.00    79   61.1                                                                             23.8                                                                             8.3                                                                              5.1                                                                              0.3                                                                              1.3    1.08                             CYCLE 2   (AVERAGE)                                                                             41                                                          CYCLE 3   (AVERAGE)                                                                             20                                                          EXAMPLE 1                                                                     CYCLE 2   (AVERAGE)                                                                             64                                                          CYCLE 3   (AVERAGE)                                                                             68                                                          EXAMPLE 2                                                                     CYCLE 2   (AVERAGE)                                                                             66.5                                                        CYCLE 3   (AVERAGE)                                                                             58.7                                                        EXAMPLE 3                                                                     CYCLE 2   (AVERAGE)                                                                             60.3                                                        CYCLE 3   (AVERAGE)                                                                             66.8                                                        __________________________________________________________________________

Results are also shown in Table 2.

As is evident from the data in Table 2, good conversion, yield of octeneand low branchiness of the octene produced is generally obtained in thefirst cycle with the NiO on SiO₂ catalyst, but the % conversion of thefeedstock over the average residence time is found to markedly drop offin the second and third cycles. This is in sharp contrast to the %conversion of the feedstock over the average residence time achieved incycles 2 and 3 using the catalysts prepared in accordance with Examples1, 2 and 3.

The following comparative example illustrates that the use of theprecipitation technique for depositing the NiO precursor material on thesupport surface is critical to achieve suitable catalytic activity.

COMPARATIVE EXAMPLE 7

A 28% by weight NiO on SiO₂ catalyst was prepared by impregnating thecatalyst with a water soluble nickel salt.

A nickel nitrate solution consisting of 67.38 g nickel nitratehexahydrate in a volume of water to make 750 cc was added to 35 gCab-O-Sil SiO₂. The catalyst was dried at 120° C. in a muffle furnacewith air flowing at 200 cc min. After drying, the impregnated materialwas calcined at 500° C. in a muffle furnace for 16 hours with airflowing at 200 cc/min.

The catalyst was found by microscopic examination to contain discretecrystallites or agglomerates predominantly of NiO present as adiscontinuous phase in regions of the support surface.

The calcined catalyst was used to attempt to polymerize transbutene-2 inaccordance with the procedure of Example 4. None of the starting feedmaterial was found to have been converted in contrast with the goodresults achieved in Cycle 1 in Comparative Example 6.

What we claim is:
 1. A process for preparing a mixture of isomericoctenes having an average of less than about 1.6 side chain methylgroups per molecule which comprises contacting butene at a temperaturebetween about 150° to 275° C. and under pressure with a catalystcomprising NiO present as a continuous, disperse substantial monolayeron the surfaces of a silica support having a surface area within therange of about 100 to 450 m² /g, said support further containing anoxide of a trivalent metal selected form the group consisting ofaluminum, gallium and indium, the quantity of said NiO present on saidsupport being in the range of form about 0.07 to 0.12% by weight persquare meter of support surface area, and the weight ratio of NiO totrivalent metal oxide present in said support being at least about 4 to1, and recovering said octene product.
 2. The process of claim 1 whereinthe weight ratio of said NiO to said trivalent metal oxide is within therange of 4:1 to 100:1.
 3. The process of claim 2 wherein the weightratio of said NiO to said trivalent metal oxide is up to 30 to
 1. 4. Theprocess of claim 3 wherein the weight ratio of said NiO to saidtrivalent metal oxide is at least about 8 to
 1. 5. The process of claim1 containing form about 21 to about 35% by weight of said NiO and fromabout 1 to bout 5% by weight of said trivalent metal oxide.
 6. Theprocess of claim 1 wherein said support has a surface area within therange of from about 200 to 400 m² /g.
 7. The process of claim 6 whereinsaid trivalent metal oxide is aluminum oxide.
 8. The process of claim 6wherein said trivalent metal oxide is gallium oxide.
 9. The process ofclaim 1 wherein said butene is selected from the group consisting oftransbutene-2, butene-1 and cisbutene-2.
 10. The process of claim 9wherein said butene is transbutene-2.